1. Field of the Invention
The present invention relates to a defect detector for detecting defects included in a detection signal of information recorded on a storage medium, and more specifically to a defect detector which detects defects in the detection signal having a lower level than a predetermined level that enables the detection signal to function as a correct detection signal.
2. Description of the Prior Art
There are broadly known information reproduction apparatuses, such as a Compact Disc player, which reproduces information recorded on a storage medium by irradiating a light beam such as a laser beam thereon and receiving the reflected light beam to obtain recorded information. In reproduction from the above optical disc, if defects such as scratches, stains or the like exist on the optical disc, the detection signal obtained from such portions of the optical disc may frequently include defects (i.e., inappropriate portion) in the detection signal (hereinafter simply referred to as "defect"). The defect generally includes two types, i.e., a relatively long defect and a relatively short defect. The long defect has the detection signal level lower than a predetermined level and lasting relatively long period of time. The short defect results from stains, fingerprints and the like on the optical disc and lasts relatively short period of time (e.g. observed as a wedge-shape in the detection signal waveform).
A defect detecting method employed on a conventional information reproduction apparatus operates in the following manner. First, an upper envelop of the detection signal is separately held by a peak-holding circuit of a long time constant and a peak-holding circuit of a short time constant. Then, the output signal of the peak-holding circuit of a short time constant is subtracted from the output signal of the peal-holding circuit of a long time constant, and the resultant signal is compared with a preset threshold level for the defect detection. If the resultant signal is lower than the preset threshold, it is judged that the defects exist.
By the above mentioned method, the relatively long defect can be readily detected because the time period in which the output signals of the peak-holding circuits of two types differ from each other is long. However, in the case of relatively short defect, the output signals of the peak-holding circuits differ only for a short period of time and the difference itself is small, and hence it is difficult to reliably detect such a short defect, especially a wedge-shaped defect. If the threshold is set at a lower level to overcome the above-mentioned problem, correct portions of the detection signal may be erroneously judged as defects and the defect compensation operation may be performed even if actually it is not the defect, thereby disturbing correct information reproduction.